The invention relates to a switching arrangement for telephone subsets using key-controlled selection units fed through charge storages via their connection lines in telecommunication systems.
An example of an arrangement of the general type is described in West German Unexamined Patent Applications 2323559 and 2345508. In addition, a number of other arrangements are known wherein semiconductor switches are employed instead of relays. However, as a rule such subsets cannot be used without limitations. For example, when such a subset is put into service, a loop and speech circuit is completed which extends over the two-wire connection line. The resulting current flowing through the connection line is recognized in the switching center and the connection line involved is connected to the continuing switching facilities. Certain conditions must be met by the monitoring circuit and the subsets in order for the switching center to recognize a closed loop without ambiguity. This is particularly true if monitoring circuits are employed with high-impedance line feeding, so that only weak monitoring currents flow through the line. If such monitoring circuits are used, the driving-point impedance of the subset must be comparatively low for a given line resistance. However, this condition is not satisfied if semiconductor switches are employed as contacts for the pulsing or for the changeover from the speaking to the dialing phase, and vice versa. The same is true if bistable relays are utilized to save energy, since normally one cannot prevent these relays from being changed over through initial current surges or the like, which results in the opening of the loop circuit necessary for the monitoring in the switching center.
In view of the foregoing, special measures are needed to bring bistable relays, if employed, to the initial position. For example, a suitable arrangement for accomplishing this is described in West German Examined Patent Application 2258839. Such a solution, however, requires additional cradle switches or switch hooks and additional connecting points for the selection unit, so that it is not capable of interworking directly with other prior art selection units.
Accordingly, it is an object of the invention to provide a switching arrangement for subsets that can operate with only three connection points for the selection unit and without the need for additional contacts.